1. Field of the Invention
The invention generally relates to evaporative emission control system and method, and, more particularly, to evaporative emission control system and method in which fuel vapor collected in a canister is drawn into an intake passage of an internal combustion engine for disposal in the engine.
2. Description of Related Art
For example, an evaporative emission control system as disclosed in Japanese Laid-open Patent Publication No. 6-58197 is known which includes a canister for collecting fuel vapor generated in a fuel tank, and a purge control valve that communicates the canister with an intake passage of an internal combustion engine as needed. In this system, when the purge control valve is opened, the intake manifold vacuum is fed to the canister so that fuel vapor collected in the canister is drawn along with air into the intake passage. Thus, the known system is able to dispose of fuel vapor generated in the fuel tank without releasing the same to the atmosphere.
According to a method as disclosed in the above-identified patent publication, the system detects an open failure of the purge control valve (which occurs when the valve is stuck in the open state) and a close failure (which occurs when the purge control valve is stuck in the closed state) while distinguishing these two types of failures from each other. In this method, the system performs no special process when it detects a close failure of the purge control valve, and performs control for correcting the air/fuel ratio while stopping learning of the air/fuel ratio when it detects an open failure of the purge control valve. The air/fuel ratio correction control includes the steps of estimating the flow rate of purge gas from the engine speed and other parameter(s), and correcting the air/fuel ratio based on the estimated flow rate, as disclosed in the above-identified publication. Thus, the known control method as described above makes it possible to correct the air/fuel ratio by some degree in view of an influence of purge gas that arises from an open failure of the purge control valve, and reduce or suppress fluctuations in the air/fuel ratio in the event of the open failure.
In the known control method, when an open failure occurs in the purge control valve, learning of the air/fuel ratio is stopped, and the air/fuel ratio is corrected by using only the estimated flow rate of the purge gas as a parameter. With the air/fuel ratio corrected by using only the estimated flow rate of the purge gas as a parameter, however, the corrected air/fuel ratio does not accurately reflect changes in, for example, the fuel concentration of the purge gas. It is thus difficult for the known control method as described above to achieve highly accurate air/fuel ratio control when the purge control valve is at fault.